1. Field of the Disclosure
The present disclosure relates generally to a transceiver of an electronic device, and more particularly, to a calibration apparatus and method for minimizing distortion of a signal of a wireless transceiver.
2. Description of the Related Art
Technologies that use a frequency mixer to convert a baseband signal or an intermediate frequency signal into a Radio Frequency (RF) signal, or to convert an RF signal into a baseband signal or an intermediate frequency signal, have been widely utilized in the wireless communication field.
However, a Local Oscillator (LO) leakage signal (or carrier leakage) may act as an interference signal in the frequency mixer, and thus, the required wireless communication standard may not be satisfied or the modulation accuracy of a communication signal may deteriorate. The carrier leakage may occur when an LO signal leaks due to a parasitic capacitance component of the frequency mixer or may occur when a frequency component of a Direct Current (DC) component existing in a baseband circuit flows into the LO signal. The leakage of the LO signal may be calibrated by applying a DC offset signal that corresponds to an inverse of the LO leakage signal to an input of the frequency mixer. The DC offset may be embodied in a Digital-to-Analog Convertor (DAC) having a resolution of a predetermined bit length, and an algorithm for determining a DAC control code for obtaining an optimal (that is, minimized) carrier leakage, is needed. For example, when the resolution of a DAC for calibration is N bits, as many as 2N events may occur. The frequency mixer of a transceiver is formed of an I-path and a Q-path, and an I-DAC and a Q-DAC exist as a DAC for calibration and thus, the number of events for selecting an optimal control code may be 22N (=2N×2N).
In addition, I/Q mismatch may occur in the frequency mixer. The I/Q mismatch refers to a mismatch of a size (or gain) and a phase between transfer characteristics of the I-path and the Q-path of the front end including the frequency mixer of the transceiver, and the I/Q mismatch may generate an image signal of a modulation signal and may deteriorate the quality of a transmission signal.
To calibrate the I/Q mismatch, a calibration circuit corresponding to an inverse of the frequency mixer, which may be modeled using a size (g) and a phase (Φ), may be used. The calibration of the size and the phase may be executed based on a method of searching for a calibration code having a determined bit length from a digital domain. This may be executed in the same manner as the method of searching for an optimal control code for calibration of carrier leakage.
In addition, in a wireless receiver, a high frequency interference signal may act as noise in a baseband by a 2nd order non-linear component (or 2nd harmonic signal or 2nd order intermodulation distortion (hereinafter referred to as a 2nd order intercept-point (IP2))) generated from a frequency down-conversion mixer for converting an RF signal into a baseband signal. To calibrate the above, an IP2 calibration procedure is needed. The IP2 calibration may also be executed through a process of searching for an optimal control code that minimizes an IP2 component of an I/Q frequency mixer. For example, the receiver requires a function of calibrating the I/Q mismatch and IP2.
A function associated with an amount of carrier leakage based on a control code (I-code and Q-code), that is, an I-DAC and a Q-DAC, for calibration of carrier leakage, generally has two characteristics. First, the size of carrier leakage may show a monotonic increment or monotonic decrement as an I-code value or a Q-code value increases, and is provided in a form of a convex quadratic parabola. Second, an amount of carrier leakage is observed by changing an I-code by one for each time, so as to detect an I-code (Imin) that minimizes carrier leakage. In this instance, Imin may be changed based on a Q-code. The characteristic may be identical in the case of a Q-code that minimizes carrier leakage. That is, Imin|Q=Q1≠Imin|Q=Q2 and Qmin|I=I1≠Qmin|I=I2. Therefore, in the case of an I-DAC and a Q-DAC having N-bit resolution, when the search for a code is executed with respect to the number of combinations of all codes (=2N×2N) so as to detect a code combination (Iopt and Qopt) that minimizes carrier leakage from among all code combinations of the I-code and the Q-code in two-dimensions, the time expended to search for the code may be great. For example, when N=8, 65,536 (=28×28) searches may be executed to detect Iopt and Qopt.
As a method of improving a code searching time, a method of determining a Q-code, detecting an Imin code that minimizes carrier leakage by repeatedly changing an I-code, fixing an I-code based on the detected Imin, and detecting a Qmin code that minimizes carrier leakage by repeatedly changing a Q-code, may be used. Due to the I/Q dependency problem in which an Imin and an Qmin are affected by a Q-code and an I-code, respectively, a method of fixing a Q-code as a Qmin and repeatedly detecting an Imin that minimizes carrier leakage by changing an I-code and repeats the process with respect to the Q-code, may be used. In this instance, at least four code search processes are required (I-code→Q-code→I-code→Q-code→ . . . ). When the number of repeated searches is Nrepeat, a total of (Nrepeat×2N) searching time is required. For example, when N=8 and the search is repeated only 4 times which is a smallest number of repeats (Nrepeat=4), (4×28=1,024) searches may be required. However, the method still requires a large number of searches and there is a large probability of failing an optimal code Iopt and Qopt when I/Q code searching is insufficiently repeated.
As described above, a code searching time is long due to the characteristics of a function of a control code and I/Q dependency, when the search for an optimal I/Q code for calibration of carrier leakage, the search for an optimal I/Q code for calibration of I/Q mismatch, and the search for an optimal I/Q code for calibration of IP2 are executed.